;;        order                     balance
;;  
;;   Peter Paul Mary        (100 + 10 - 20) / 2 = 45
;;   Peter Mary Paul        (100 + 10) / 2 - 20 = 35
;;   Paul Peter Mary        (100 - 20 + 10) / 2 = 45
;;   Paul Mary Peter        (100 - 20) / 2 + 10 = 50
;;   Mary Peter Paul        100 / 2 + 10 - 20   = 40
;;   Mary Paul Peter        100 / 2 - 20 + 10   = 40
